1. Field of the Invention
This invention relates to minimizing defects in a painted coating on a composite material article. This invention more particularly relates to improvements that can be especially used in applying a clear finish coat over a color coating on a glass fiber/resin composite material article.
2. Description of the Prior Art
Before describing the prior art, it should be mentioned that fiber glass/thermosetting resin composites present special problems in painting, as compared to metal. Painting techniques that are satisfactory on metal, can produce blistering and cratering when used on glass fiber/resin composites. More specifically, the currently used clear coat final finishes used on automotive metal parts are cured at temperatures of about 250.degree.-325.degree. F. Use of such relatively high curing temperatures on composite parts can expand subsurface air bubbles in a composite part, to blister or crater the parts's surface. In view of such problems, use of paint systems that cure at lower temperatures have been tried. However, use of the lower cure temperatures produces still other problems. This invention provides means for solving one of those other problems that is particularly aggravating. The term "paint system" is intended to include all coatings used to form a "painted" surface on a part, including both a pigmented primer coat and a clear finish coat as well as the color coat. Accordingly, a clear finish coat that lies on a color coat of paint is still considered to be a "paint" for purposes of this invention, even thought the clear finish coat contains no pigment.
More specifically, this invention is particularly beneficial in minimizing defects in a clear resin finish coat of painted composite material automotive products, especially such products made of glass fiber-reinforced thermosetting resin. The defects are minimized by prolonged preheating of the composite material at a particular temperature, and then painting and curing the finish coat while the composite material remains substantially at that temperature. While the prior art contains references to preheating all types of products prior to painting, the prior art offers no suggestion of my process, or its benefits when painting composite materials.
As indicated above, the preheating of articles prior to painting is not new. A number of patents teach a variety of processes that involve heating of an article prior to and/or during painting, primarily to accelerate the drying process. For example, U.S. Pat. No. 0,665,747 Martin preheats both the article and the paint, and applies the paint in a heated chamber. U.S. Pat. No. 2,763,575 Bede heats the paint to self-pressurize the paint, then sprays the hot paint in a heated chamber. U.S. Pat. No. 5,130,173 Barten et al. discloses painting automotive radiators and condensers faster by preheating them and spraying them with preheated paint.
U.S. Pat. No. 2,861,897 Hendrixson describes a technique for applying an extra thick coating of paint to a metal article. The article is heated to a temperature just below the boiling point of the solvent in the paint. The article is passed through a solvent reflux (i.e., hot) zone of a columnar paint chamber, and sprayed with hot paint. The paint is then allowed to dry in the spray chamber, using latent heat in the metal article. The article exits the spray chamber by passing through the reflux zone quickly, to avoid rinsing off the paint layer. U.S. Pat. No. 3,042,547 Picket discloses that chlorinated paint solvents have coalescence and flow problems. Picket solves them using an apparatus and process analogous to that of Hendrixson. The part is apparently preheated in a solvent re-flux zone, and painted with jets of paint. The article is then dried at room temperature using its latent heat before it exits the apparatus. U.S. Pat. No. 3,073,721 Pokorny also recognized that chlorinated solvents do not coalesce and level off a paint film in an acceptable manner. Like Picket, Pokorny proposes preheating the workpiece in a re-flux zone of the paint solvent. The workpiece is preheated to a temperature just below the vaporization temperature of the paint solvent. The paint is heated to a temperature above its solvent vaporization temperature, and is then applied as a jet onto take preheated workpiece. Because the workpiece is cooler than the vaporization temperature of the paint solvent, the paint coalesces and levels off. However, the paint dries rapidly, whereupon the workpiece can be removed through the re-flux zone without rinsing off the paint.
The latter type of high temperature painting process may be satisfactory for metal. Metal workpieces can ordinarily safely withstand higher painting and drying, or curing, temperatures. It is not unusual to dry and/or cure metal automotive body parts at temperatures of about 250.degree.-325.degree. F., to accelerate the painting process. I have noted that the metal part absorbs heat rapidly, and releases it to the paint coating. I have recognized that this action substantially evaporates solvents substantially throughout the thickness of a paint coating before the surface of the paint coating hardens, i.e., dries.
On the other hand, none of the foregoing patents addresses methods for improving the quality of the specular, i.e., gloss, finish of paint on plastics, especially automotive SMC or RTM composites. It is noted that U.S. Pat. No. 3,911,178 McDowell et al. concerns painting injection molded thermoplastic automobile parts. McDowell et al. state that such parts inherently have a waxy surface residue that causes "fisheying". McDowell et al. solve the problem of "fisheying" by preheating the thermoplastic part to about 130.degree. F., applying a barrier clear coat, partially curing the barrier clear coat, preheating the part to about 130.degree. F. and applying a color coat. Thereafter, the color coat is fully cured at about 180.degree.-260.degree. F.
Different problems, from "fisheying", exist when painting a plastic part made of thermosetting resin. Among such thermosetting resin parts are glass fiber reinforced parts such as sheet molded (SMC) composite material parts and resin transfer molded (RTM) composite material parts. The current paint finishes for automotive body parts, whether of metal or composites, involve a lamination of coatings. The lamination includes a primer coating, a color coating, and a clear resin finish coat. The coatings are usually sprayed on as a liquid, utilizing a vaporizable solvent, and then dried. Drying involves solidification of the coating by vaporization of the solvent used to liquify it. However, the clear resin finish coat, is more than just dried. The clear resin finish coat is cured in a lengthy heating, that provides a durable lustrous finish having an apparent thickness or "depth" to the underlying color coat. Such laminar finishes can be readily applied to metal. However, when such paint systems are applied to an SMC and an RTM composite material product, special problems develop. The primer and color layers of such laminar paint systems are apparently formed fairly satisfactorily on a SMC or RTM composite product, with an exception hereinafter described. Unfortunately, the clear finish coat does not form nearly as well. The spraying of the clear finish coat onto the primer and color coats is not the problem. The problem involves curing of the clear coat finish. As indicated above, if the clear finish coat on composite substrates is cured at temperatures "normal" for curing it on a metal substrate, unique problems arise. Subsurface air bubbles in the composite, especially an RTM composite, can permanently expand, and even "pop", the composite's surface. This results in the substrate surface finish having smooth or cracked bumps, and even uncoated craters. One technique for reducing this problem is to preheat the composite part at cure temperatures, to reveal potential problem spots. After cooling the composite part, the revealed problem spots are repaired. Then, the clear finish coat is applied, hoping that no further such spots will reveal themselves when the clear finish coat is cured. An additional and/or alternative approach to solving this problem resides in simply curing the clear finish coat at a lower temperature (than used on metal). For example, the clear finish coat on SMC and RTM composite material products is now often cured at temperatures of approximately 160.degree.-180.degree. F.
Surprisingly, even at such low curing temperatures, the cured clear finish coat can still exhibit a blistered and/or cratered appearance. However, I have found that this latter blistering and/or cratering is in the clear finish coat, itself. It is not in the substrate. Hence, preheating to curing temperatures does not even reveal potential trouble spots for repair. I have found that this latter type of blistering and/or cratering is due to a different mechanism than the blistering and/or cratering referred to in the preceding paragraph. I have found that blistering and/or cratering in the clear coat finish is related to porosity, not air bubbles, in the SMC or RTM substrate. Porosity in the surface of an SMC and RTM composite material part exists even after the part has received a primer and color coating of paint. Moreover, the clear coat blistering and/or cratering problem is aggravated in RTM articles having an thick foam inner layer. The clear coat cratering and blistering problem may even be still more aggravated in RTM articles in which the thick foam inner layer has a metal reinforcement.
I have discovered that the cratering and blistering in and under the clear coat is associated with registered porosity in the composite article and in its primer and color coatings. Further, I have discovered that a simple modification of the clear coat painting technique can overcome the adverse effects of this registered porosity.